The advance of CDMA technology has brought about new ways of using more effectively radio frequencies on the whole spectrum, which in comparison to TDMA and FDMA systems provides a possibility for a greater number of users per a frequency channel, for example, a faster power regulation, a more effective use of data transmission rates and a better connection between a base station and a subscriber terminal equipment. The possibility to use different data transmission rates in the subscriber terminal equipment adds to the capacity of the system as when a speech connection is not in use or when the amount of data to be transferred is small, the capacity of the cellular radio system can be increased at a smaller data transmission rate, interference being diminished for other users. In future CDMA standards, data transmission rate can be changed specifically for each frame on the basis of speech activity so that the rate in a single data transmission frame is the same, while a frame generally comprises 16 transmission time slots.
The data transmission rates to be transmitted, of which there are typically four, are divided at random to said sixteen different transmission time slots on the basis of a long spreading code, whereby the greatest data transmission rate (e.g. 9,600 kbps) uses all 16 time slots, when the data transmission rate is half of the greatest data transmission rate, eight time slots out of sixteen are used, when the data transmission rate is fourth of the greatest data transmission rate, four time slots out of sixteen are used, and the smallest data transmission rate is eighth of the greatest data transmission rate and only two time slots out of sixteen transmission time slots are in use.
In the base station the transmission time slots used by all the data transmission rates can be calculated and detected when the used long spreading code is known. Different data transmission rates also share a feature that all the transmission time slots of the next smallest data transmission rate by one are the same (that is, the greatest data transmission rate has eight of the same transmission time slots as a lower data transmission rate.)
When different data transmission rates are being used, the base station should identify the transmitted data transmission rate, which is not very easy. Various interferences and fadings in the radio channel make the identification of the data transmission rate difficult and it is difficult to calculate the data transmission rate from all data transmission rates to Viterbi decoding and a complicated equipment is required for it. In prior art solutions, a parallel Viterbi decoding is used separately for each data transmission rate. In that case the possible calculated information of a received signal on the data transmission rate is not utilized in any way. All the data transmission rates are examined one by one and the decision of the data transmission rate of an incoming signal is made specifically for each frame by means of other check routines. Viterbi decoding by parallel processing requires a complicated circuit implementation and thus all solutions that simplify implementations at the component level are more than welcome. The CDMA system generally uses a Walsh-Hadamard transform which contains calculated information on the signal noise ration, which is explained in more detail in Finnish Patent Application 956359, and on the data transmission rate partly based on it.